Desert Battle problem

It seems that Desert Battle isn't quite right.

When all enemies are eliminated in level 2 it doesn't move onto level 3. I thought maybe I had to destroy the helicopter in the last screen of level 2, or there were some combatants hiding out, but after many attempts it seems everybody is dead and the big copter cannot be destroyed.

An hour or so later I came up with a bright idea... I found the same game on another site and it does continue to level 3 after all the human enemies are destroyed in level two.

Must be something wrong with the majorgeeks' copy.